The present invention relates to a single-chip microcomputer having a semiconductor substrate being loaded with peripheral equipment including a CPU (central processing unit), a ROM (read-only memory), a RAM (random-access memory), a timer, a serial input/output, an A/D converter and so forth. In particular, the present invention relates to a microcomputer loaded with a PROM (programmable non-volatile semiconductor memory), and to a method of testing a read-out operation of the data of the PROM.